[EKS] Webhook as a Lambda

We can run webhooks as lambdas instead of a persistence service continually running in cluster.

I would like to add this as a deployment option.
Links for reference:
https://github.com/aws-samples/amazon-api-gateway-mutating-webhook-for-k8
https://github.com/nbrandaleone/eksClient/
https://github.com/kelseyhightower/denyenv-validating-admission-webhook

The webhook pkg just needs to be broken out here:
https://github.com/aws/aws-app-mesh-inject/blob/master/pkg/webhook/server.go#L132

so we can handle http and lambda in the same fashion.

Looking for feedback before I start.

Start with the referenced pkg/webhook/server.go implementation in aws-app-mesh-inject and review the linked API Gateway and EKS webhook examples. Determine how the webhook can support both HTTP and Lambda deployment options, with the deployment option documented and working consistently for both paths.

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